Abstract For over half a century, prostate cancer research has focused on the Androgen receptor (AR), its role in the initiation of the disease and progression to the highly metastatic stage, castration resistant prostate cancer (CRPC). While AR antagonists such as enzalutamide or androgen- synthesis inhibitor abiraterone dampen AR functional activity, these are often ineffective long term, as the recalcitrant disease recurs within 2-3 years. CRPCs not only thrive under low or castrate levels of androgen, but also fastidiously maintain functional AR. Consequently, anti- androgen-resistance has become one of the most vexing problems in prostate cancer therapy. Despite intensive efforts, targeting co-factors that regulate AR and its splice variant, AR-V7 transcription, directly and efficaciously, with small molecule inhibitors has not yet been achieved. We uncovered that AR recruits the oncogene ACK1, a non-receptor tyrosine kinase, to regulate its own expression- setting up a pathological positive feedback loop in androgen deprived condition. Mechanistically, activated ACK1 modifies chromatin via phosphorylation of the histone H4 at a novel site, tyrosine 88 (pY88-H4), upstream of the AR gene at three sites AREM1-3, to promote transcription. Conversely, reversal of this pY88-H4 histone modification by treatment with a novel ACK1 inhibitor, (R)-9bMS, or overexpression of the H4Y88F mutant significantly suppressed transcription of the full length AR as well as AR-V7 splice variant, consequently downregulating expression of AR-target genes, such as PSA. Moreover, we demonstrate that WDR5/MLL2 histone-Lysine N-Methyltransferase complex, interact with the pY88-H4 epigenetic marks, deposit the transcriptionally activating H3K4 tri-methyl marks in trans, thus uncovering a novel mode of epigenetic regulation at the AR locus. Based on these extensive preliminary data, we hypothesize that inhibition of ACK1 epigenetic regulator activity that suppresses AR transcription and block production of AR splice variants will be critical to combat enzalutamide- and abiraterone-resistant CRPCs. To address this hypothesis, we will pursue the following aims: Aim 1. Assess the functional role of pY88-H4 deposition sites AREM1-3 on AR/AR-V7 transcription in CRPCs. Aim 2. Examine genome wide MLL2/WDR5/ASH2L and pY88-H4 co-association and its role in driving AR/AR-V7 expression and CRPC progression. Aim 3. Detail characterization of ACK1 inhibitor (R)-9bMS to overcome enzalutamide & abiraterone-resistance and ADME/DMPK studies.